47th Annual AAPG-SPE Eastern Section Joint Meeting

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Critical questions of hydrocarbon generation: fluid composition and maturation determined from new techniques using maturity proxies


Assessment of unconventional shale reservoirs and potential source rock intervals rely on estimates of thermal maturity and extent of kerogen transformation to hydrocarbons. Vitrinite reflectance (%Ro), bitumen reflectance (%Bro), TMAX, hydrogen index (HI), transformation ratio (TR), and calculated Ro (eq) values based upon time/temperature relationships are used to determine maturity indices. Problems can arise in applicability and interpretation of maturity proxies that are used to estimate hydrocarbon generation. Vitrinite suppression, bitumen inclusion in S2 peaks, heterogeneity in organic richness, and inaccurate time/temperature relationships can contribute to problematic maturity estimates. Experimental closed-system pyrolysis and modeling suggest that the TR-HI relationship is non-linear and deviates from a linear relationship in areas with oil-prone and elevated original HI source rocks. Potential unconventional reservoirs containing excellent oil-prone kerogens, have calculated transformation ratios (from HI values) that significantly underestimate the conversion of organic matter to hydrocarbon. Combining a new approach in the assessment of original HI in heterogeneous organic facies with a TR calculation that accounts for the curved relationship would result in a maturity proxy that is more reliable in hydrocarbon generation estimates. Source rock pyrolysis data from the New Albany Shale (NAS) in the Illinois Basin was used to evaluate the new techniques. Preliminary results indicate that calculated TR values expanded the range of incipient oil generation by 10 – 25 percent. In addition, zones of increased maturity within the NAS were postulated in the central/northern La Salle anticlinorium by HI maturity proxies and confirmed by TR estimates. Preliminary modeling suggest that at TR of roughly 50% is equivalent to a maturity estimate of 0.60 – 0.65 %Ro, early oil generation window. Linkage of the TR estimates with predicted fluid compositions was attempted using KINEX and GENESIS Zetaware modeling. In the Illinois Basin, preliminary results indicate intervals of predicted high-volatile oil in the southern Wabash Valley Fault zone, Fairfield Basin and southern La Salle anticlinorium.